1. Field of the Invention
The present invention relates to a converter for converting single-ended signals to differential signals, which is necessary for receiving an output of a single-ended signal circuit by use of a differential-end signal circuit, in particular relates to a converter of converting single-ended signals to differential signals within a monolithic integrated circuit (IC).
2. Description of the Related Art
In an analogue circuit, a single-ended signals to differential signals converter for converting single-ended signals to differential signals is a very important basic circuit. FIG. 12 shows an existing single-ended signals to differential signals converter employing a differential amplifier.
This circuit operates as follows. That is, a single-ended input signal inputted from an input terminal is inputted to a base of a transistor Q1 through a coupling capacitor C1. On the other hand, a base of a transistor Q2 is grounded through a capacitor C2 of identical capacitance with that of the capacitor C1. The transistors Q1 and Q2 form a transistor pair coupled to each other through an emitter and from collectors of the transistors Q1 and Q2, a pair of differential signals converted from the input signal are outputted.
If this circuit operates ideally, the differential output does not contain common mode components. Accordingly, outputs of the differential signals are summed up to be a definite potential. In an actual circuit, however, there is a problem that common mode components remain between the outputs of differential signals, as a result of this, the summation of outputs of differential signals is not a definite potential. This is caused by a fact that the impedance of a bias current supply I.sub.0 of the emitter-coupled transistor pair is not infinite in an actual circuit.
FIG. 13 is an existing single-ended signals to differential signals converter employing a transformer. In this method, a separate transformer 133 other than the first circuit 131 and the second circuit 132 is employed to implement single-ended signals to differential signals conversion. Here, to a primary coil L1 of the transformer 133 a single-ended output terminal of the first circuit 131 is connected, and to a secondary coil L2 of the transformer 133 a differential input terminal of the second circuit 132.
However, in general a transformer has an area far bigger than that of a transistor or a resistor incorporated in an integrated circuit (IC). Accordingly, there is a problem that an area of circuits as a whole becomes large.